I want to play DoTA 2 with someone, and they have it in their library (from the survey) but they never got a key. They want to buy the DoTA 2 pack on sale so they can play it, but since it's in their library, they can't purchase it (even though they can't play it because they don't have a key.) :C Is there any way around this?

When I unlocked DoTA 2 from the survey I didn't get a seperate key; rather, I got a Steam news pop up saying that I got into the beta and an email from Valve saying that I got into the beta. It is quite possible that they already have the game, assuming that they did indeed get into the beta via the survey.